Low voltage input current mirror circuit and method

ABSTRACT

A low voltage current mirror circuit (also referred to as a bias circuit) for establishing a plurality of bias voltages from an input current supplied to an input terminal of the circuit includes an input stage, a current stage connected to the input stage, a feedback stage connected to the current stage, a reference bias stage connected to the feedback stage and the current stage. The circuit establishes first and second bias voltages suitable for biasing current sources of a first type, and third and fourth bias voltages suitable for biasing current sources of a second type complementary to the first type. The bias voltages track the input current over variations in at least one of process, temperature and power supply voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of the U.S. Non-ProvisionalApplication entitled “Low Voltage Input Current Mirror Circuit andMethod,” Ser. No. 09/897,045, filed Jul. 3, 2001, which claims priorityto the U.S. Provisional Application entitled “Low Voltage Input CurrentMirror,” Ser. No. 60/221,835, filed on Jul. 28, 2000, and also to theU.S. Provisional Application entitled “Universal Cable Tuner RF FrontEnd Chip,” Ser. No. 60/215,850, filed Jul. 3, 2000, all of which areincorporated herein in their entireties by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to bias circuits, and moreparticularly, to such a bias circuit for establishing bias voltagessuitable for biasing current sources.

2. Related Art

FIG. 7A is a circuit diagram of a known, simple current mirror includingan input diode M31 and a current source Field Effect Transistor (FET)M32. The simple current mirror simply replicates (perhapsproportionately) the input diode current I_(IN2) as an output currentI_(OUT2). While this circuit is simple, a problem can arise because thedrain-source voltage of FET M31 is not necessarily equal to thedrain-source voltage of FET M32. This causes the current I_(OUT2)flowing through FET M32 to be different from the current I_(IN2) flowingthrough diode M31. This is especially the case for devices havingrelatively short channels (also referred to as short-channel devices),such as sub-micron devices.

FIG. 7B is a circuit diagram of a known cascode current mirror used tosolve the above-mentioned problem. The cascode current mirror keeps thedrain-source voltages of both FETs M33 and M34 the same. However, thevoltage at the top of FET M35 (that is, on the drain of FET M35) can berelatively high, perhaps more than ½ the power supply voltage VDD.Therefore, changes in voltage VDD cause significantly largercorresponding changes in input current. All of this amounts to a circuithaving the disadvantage of very high power supply sensitivity (that is,an undesired sensitivity to power supply voltage variations).

FIG. 7C is a circuit diagram of a self-biased current mirror used toovercome the above-mentioned power supply sensitivity. The currentthrough M42 is basically the voltage across diode M41 divided by theresistance of R10. This current can then be mirrored to the outputthrough the p-type Metal Oxide Semiconductor (PMOS) devices M44-M46.Such self-biased reference circuits also need a start-up circuit toensure they attain a proper operating state. The circuit of FIG. 7Ctends to have the disadvantage that currents in the circuit tend to varyin undesired or wrong directions over process and temperaturevariations. Also, the input current can not be conveniently adjusted.

FIG. 7D is a bandgap circuit using parasitic bipolar transistors in aComplementary Metal Oxide Semiconductor (CMOS) substrate to createcontrolled reference voltages. One voltage goes as delta-VBE and theother goes as KT/q multiplied up. Since the temperature coefficients ofeach of these voltages go in opposite directions, a temperatureindependent voltage can be achieved. However, bandgap references tend torequire a start-up circuit to ensure proper operation thereof. Also, thebandgap circuit is not space-efficient because of the large arearequired by the PNP transistors used in the circuit. PNP transistors arelateral (not vertical) devices with poor beta and very low maximumcurrent.

There is a need therefore for an improved bias circuit that overcomesall of the above-mentioned shortcomings and disadvantages of knowncircuits.

SUMMARY OF THE INVENTION

Summary

The present invention overcomes the above-mentioned shortcomings anddisadvantages of know circuits. The present invention is directed to alow voltage input current mirror circuit (also referred to as a biascircuit) for establishing a plurality of bias voltages from an inputcurrent supplied to an input terminal of the bias circuit. In oneembodiment, the circuit includes an input stage adapted to establish afirst bias voltage at the input terminal in response to the inputcurrent. The circuit further includes a current stage adapted to producea bias current and a main mirror current each proportional to the inputcurrent in response to the first bias voltage and a second bias voltage.The circuit further includes a feedback stage adapted to produce afeedback current proportional to the input current in response to thebias current and the main mirror current. The circuit further includes areference bias stage adapted to establish the second bias voltage inresponse to the feedback current from the feedback stage, whereby thefirst and second bias voltages track the input current over variationsin at least one of process, temperature and power supply voltage.

Another aspect of the present invention is a method of establishing aplurality of bias voltages suitable for biasing current sources from aninput current supplied to a bias circuit. The method comprises the stepsof (a) supplying an input current, (b) establishing a first bias voltagein response to the input current, (c) producing a bias currentproportional to the input current in response to the first bias voltageand a second bias voltage, (d) producing a main mirror currentproportional to the input current in response to the first bias voltageand the second bias voltage, (e) producing a feedback currentproportional to the input current in response to the bias current andthe main mirror current, and (f) establishing the second bias voltage inresponse to the feedback current, whereby the first and second biasvoltages track the input current over variations in at least one of atemperature and a power supply voltage of the bias circuit.

Features and Advantages

A. The bias circuit of the present invention is more space-efficient,physically smaller, and less complex than known bandgap referencecircuits.

B. The bias circuit of the present invention exhibits much lower thermalnoise than the bandgap reference circuit, for example, when an externalcapacitor to ground is used across an input stage of the bias circuit.

C. The bias circuit of the present invention uses an external resistorto set an input current to the bias circuit, allowing for a trade-offbetween performance and power.

D. The bias circuit of the present invention includes a shut-down stageor mechanism to selectively turn-off an input current to the biascircuit.

E. The bias circuit of the present invention generates referencevoltages compatible with complementary types of logic, such as NMOS andPMOS reference circuits.

F. The bias circuit of the present invention has low power supplysensitivity.

G. The bias circuit of the present invention produces reference currentsand bias voltages that vary only slightly with process, temperature andpower supply voltage. These variations tend to partially compensate gainvariations, without increasing distortion.

BRIEF DESCRIPTION OF THE FIGURES

The features, objects, and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify the same or similar elements throughout and wherein:

FIG. 1 is a high-level block diagram of an example low voltage inputcurrent mirror circuit (bias circuit) according to the presentinvention.

FIG. 2 is a circuit diagram expanding on the circuit of FIG. 1.

FIG. 3 is a circuit diagram of an example input circuit portionconnected to the circuit of FIG. 2.

FIG. 4A is a circuit diagram of a start-up stage or circuit according toone embodiment of the present invention.

FIG. 4B is a circuit diagram of a start-up circuit according to anotherembodiment of the present invention.

FIG. 4C is a circuit diagram of a start-up circuit according to stillanother embodiment of the present invention.

FIG. 5A is a circuit diagram of a shut-down stage according to anembodiment of the present invention.

FIG. 5B is a circuit diagram of a shut-down stage according to anotherembodiment of the present invention.

FIG. 5C is a circuit diagram of a shut-down stage according to stillanother embodiment of the present invention.

FIG. 6A is a flowchart of an example method of establishing first andsecond bias voltages from an input current implemented using the circuitof FIG. 2.

FIG. 6B is a flowchart expanding on the method of FIG. 6A.

FIG. 6C is a flowchart of an example method further expanding on themethod of FIG. 6A.

FIG. 6D is a flowchart of an example method of initially establishing aproper operation of the circuit of FIG. 2.

FIG. 6E is a flowchart of an example method of selectively enabling anddisabling the circuit of FIG. 2.

FIG. 7A is a circuit diagram of a conventional simple current mirror.

FIG. 7B is a circuit diagram of a conventional cascode current mirror.

FIG. 7C is a circuit diagram of a conventional self-biased currentmirror.

FIG. 7D is a circuit diagram of a conventional bandgap reference circuitused to create controlled reference voltages.

DETAILED DESCRIPTION OF THE INVENTION

Overview

FIG. 1 is a high-level block diagram of an example low-voltage inputcurrent mirror circuit 100 (also referred to as bias circuit 100),according to the present invention. Bias circuit 100 includes an inputcurrent source 102 for supplying an input current 104 (I_(IN)) to a maincircuit portion 106 (also referred to as circuit 106), to be describedin detail below. In response to input current 104, circuit 106establishes a first set of bias voltages VBN1 and VBN2, as well as asecond set of bias voltages VBP1 and VBP2. Circuit 106 applies biasvoltages VBN1/VBN2 to a current source 110 of a first type compatiblewith the first set of voltages. Current source 110 produces a current112 in response to bias voltages VBN1/VBN2. Similarly, circuit 106applies bias voltages VBP1/VBP2 to a current source 120 of a second typecomplementary to the first type and compatible with the second set ofbias voltages. Current source 120 produces a current 122 in response tobias voltages VBP1/VBP2. In one arrangement of the present invention,current sources 110 and 120 are respectively NMOS and PMOS cascodecurrent sources. In the art, NMOS current sources are generally referredto as current sinks, while PMOS current sources are generally referredto as current sources.

FIG. 2 is a circuit diagram expanding on bias circuit 100 of FIG. 1.Depicted in FIG. 2 are input current source 102, main circuit portion106 (depicted centrally in FIG. 2 between vertical lines 202 a and 202b), and current sources 110 and 120 (on the right side of FIG. 2). In anintegrated circuit embodiment of the present invention, main circuitportion 106 is constructed on an integrated circuit (IC) chip, and inputcurrent source 102 is external to the IC chip. In the integrated circuitembodiment, one or more current sources, such as current sources 110 and120, may be external to the IC chip, internal to the IC chip, or bothexternal and internal to the IC chip.

A first power supply rail 204 and a second power supply rail 206 supplypower to bias circuit 100. In an exemplary arrangement, first powersupply rail 204 applies a voltage VDD (for example, 3.3 Volts) to biascircuit 100, while second power supply rail 206 applies a voltage VSS(corresponding to a ground (GND) potential) to bias circuit 100.

Current source 102, connected between first power supply rail 204 and aninput terminal 208 of circuit 106, supplies input current I_(IN)(corresponding to current 104 in FIG. 1) to the input terminal. Circuit106 includes an input stage 210 connected to input terminal 208, and acurrent stage 212 connected to input stage 210. Circuit 106 alsoincludes a feedback stage 214 connected to current stage 212, and areference bias stage 216 connected to both current stage 212 andfeedback stage 214. Circuit 106 further includes a start-up stage orcircuit 218 connected between first power supply rail 204 and a terminal220 common to both feedback stage 214 and reference bias stage 216.

A brief operational overview of bias circuit 100 is now provided. Inputstage 210 establishes bias voltage VBN1 at input terminal 208 inresponse to input current I_(IN) supplied to the input stage. Currentstage 212, also connected to input terminal 208, produces a bias current222 and a main mirror current 224 in response to both bias voltage VBN1and bias voltage VBN2, such that the two currents are proportional toinput current I_(IN). In response to bias and main mirror currents 222and 224, feedback stage 214 produces a feedback current 226 proportionalto input current I_(IN). Reference bias stage 216 produces bias voltageVBN2 in response to feedback current 226. The above-described feedbackarrangement, along with other circuit characteristics to be describedlater, causes the bias voltages VBN1/VBN2 to track input current I_(IN)over variations in process, temperature, and power supply voltage (forexample, variations in VDD and VSS).

Detailed Circuit Description

A detailed circuit description of bias circuit 100 is now provided.Example bias circuit 100 depicted in FIG. 2 is constructed using n-typeMetal Oxide Semiconductor Field Effect Transistors (MOSFETs) and p-typeMOSFETs (that is NMOS and PMOS FETs). Each FET also includes a bulk (orsubstrate) connection terminal, not shown. It is assumed the NMOS FETsubstrates are connected to VSS (GND) and the PMOS FET substrates areconnected to VDD. Each FET includes drain, source, and gate or controlelectrodes. Each FET depicted in FIG. 2 includes a directional arrowidentifying the source of the FET. An arrow pointing away from the gateindicates an NMOS FET, while an arrow pointing toward the gate indicatesa PMOS FET.

Each of the FETs depicted in FIG. 2 represents an aggregate of manysmaller FETs connected together (that is, in parallel with one another)to form one, larger aggregate FET (such as FETs M1, M2, and so on,depicted in FIG. 2). An advantage of constructing such an aggregate FETis that the size and thus current carrying capability (and associatedvoltage drops produced by) the aggregate FET can be carefullycontrolled. Most of the FETs of bias circuit 100 are sub-micron devices.This means each of the smaller individual FETs used to construct anaggregate FET has a minimum channel width below one micron (for example,a channel width of 0.35 microns). For example, FET M2 includesthirty-two (32) individual FETs, each having a channel size, representedherein in terms of channel width (W) and channel length (L), ofapproximately 10 microns (W) by 0.35 microns (L).

It is to be understood the present invention can be constructed usingdevices other than FETs. For example NPN and PNP bipolar transistors ora mix of such bipolar transistors and field effect transistors can beused, as would be apparent to one skilled in the relevant art afterhaving read the description of the present invention.

Input Stage (210)

Input stage 210 includes an input NMOS FET M1 configured to operate as adiode and connected between input terminal 208 and second power supplyrail 206. The input configuration including power supply rail 204,current source 102, FET diode M1, and power supply rail 206, establishesa gate-source voltage and a drain-source voltage of FET M1 correspondingto input current I_(IN). The drain-source voltage across FET M1 alsoappears across input terminal 208 and power supply rail 206, andestablishes bias voltage VBN1 at input terminal 208. Input diode M1 is arelatively large device, and thus establishes a relatively low voltage,between 500 and 600 milliVolts (mV), for example, at input terminal 208.This relatively low voltage has the advantage of desensitizing circuit106 to fluctuations in voltage VDD.

Current Stage (212)

Current stage 212, connected to input diode M1, includes a main mirrorcurrent stage 232 for producing main mirror current 224, and a biascurrent stage 230 for producing bias current 222.

Main mirror current stage 232 includes a first NMOS FET M4 for setting avalue of main mirror current 224 and a second FET M5 connected to FET M4in a cascode configuration. FET M4 has a gate connected to inputterminal 208 and a source connected to power supply rail 206. Thisestablishes a gate-source voltage of FET M4 equal to the gate-sourcevoltage of FET M1. Cascode FET M5 includes a source-drain path connectedbetween the drain of FET M4 and a terminal 234 such that the respectivesource-drain current paths of FETs M4 and M5 are connected in serieswith one another and are connected together between second power supplyrail 206 and terminal 234. The gate of FET M5 is connected to an output(terminal 220) of reference bias stage 216, whereby the reference biasstage applies voltage VBN2 to the gate of FET M5. FET M5 operates as acascode or buffer device in connection with FET M4, to maintain apreferred source-drain voltage across FET M4, as will be furtherdescribed below. FET M4 is operated in its saturation region.

Bias current stage 230 includes a first NMOS FET M2 for setting a valueof bias current 222 and a second FET M3 connected to FET M2 in a cascodeconfiguration. FET M2 has a gate connected to input terminal 208 and asource connected to power supply rail 206. This establishes agate-source voltage of FET M2 equal to the gate-source voltage of FET M1(and FET M4). FETs M2 and M3 have their respective source-drain currentpaths connected in series with one another and are together connectedbetween second power supply rail 206 and a terminal 236. The gate of FETM3 is connected to the output (terminal 220) of reference bias stage216, whereby the reference bias stage applies voltage VBN2 to the gateof FET M3. FET M3 operates as a cascode or buffer device in connectionwith FET M2, to maintain a preferred source-drain voltage across FET M2,as will be further described below. FET M2 is operated in its saturationregion.

A goal of circuit 106 is to have FETs M2 and M4 replicate preciselyinput current I_(IN). In other words, the goal is to have FETs M2 and M4respectively set bias and main mirror currents 222 and 224 proportionalto input current I_(IN) flowing through diode M1 over process,temperature, and power supply variations. The reason for this is thatcircuit 106 uses currents 222 and 224 as reference currents for derivingfurther currents and bias voltages (for example, bias voltages VBN2,VBP1, and VBP2), and it is desirable that such further currents and biasvoltages also track input current I_(IN) over process, temperature, andpower supply variations.

When two or more FETs (for example, FETs M1, M2, and M4 in FIG. 2) have(a) equal gate-source voltages, and (b) equal drain-source voltages, theFETs produce currents through their respective source-drain currentpaths in proportion to their respective sizes. For example, when theFETs are the same size, their respective source-drain currents (alsoreferred to as drain currents) are equal. In other words, theirrespective drain currents are in the proportion or ratio of 1:1 withrespect to one another. When one FET is twice as large as the other FET,the larger FET sets a drain current twice as large as the smaller FET,and so on, assuming equal gate-source and drain-source voltages acrossthe two FETs.

Therefore, to replicate input current I_(IN) flowing through FET M1 inboth FETs M2 and M4 (that is, in bias and main mirror currents 222 and224), circuit 106

(a) sets the gate-source voltage across each of FETs M2 and M4 equal tothe gate-source voltage across M1 by circuit connection (as depicted inFIG. 2, and described above), and

(b) maintains the drain-source voltage across each of FETs M2 and M4equal to the drain-source voltage across FET M1 using theabove-mentioned feedback configuration including cascode configured FETsM3 and M5, as will be further described below.

Therefore, circuit 106 achieves the goal of matching bias and mainmirror currents 222 and 224 to input current I_(IN) (that is, ofreplicating the input current) over variations in process, temperature,and power supply.

Feedback Stage (214)

Current stage 212 supplies bias current 222 and main mirror current 224to feedback stage 214. Feedback stage 214 includes a low-voltagereference voltage stage 238 for establishing bias voltages VBP1 and VBP2in response to bias current 222 and main mirror current 224. Referencevoltage stage 238 includes a bias stage 240 for establishing biasvoltage VBP2 in response to bias current 222, and a reference stage 242for establishing bias voltage VBP1 in response to both main mirrorcurrent 224 and bias voltage VBP2. Feedback stage 214 also includes acurrent source 244, connected to both stages 240 and 242, to producefeedback current 226 in response to bias voltages VBP1/VBP2 establishedby reference voltage stage 238.

Low-Voltage Reference Voltage Stage (238)

Bias stage 240 includes first and second PMOS FETs M8 and M9 havingtheir respective source-drain current paths connected in series witheach other and connected together between first power supply rail 204and terminal 236. The gates of both FETs M8 and M9 are connected toterminal 236 (the drain of FET M9). Bias current 222 flows through FETM8 and establishes the gate-source voltage of FET M8, and thus, voltageVBP2 on the gate of FET M8. The gate of FET M8 applies voltage VBP2 tothe drain of FET M9 by direct connection, thereby minimizing the overallvoltage drop across the combined source-drain paths of FETs M8 and M9.This arrangement establishes a minimum source-drain voltage across FETsM8 and M9 required to cause the FETs to operate in saturation (asopposed to the triode region). FETs M8 and M9 operate as an aggregatediode. Bias voltage VBP2 has an exemplary value of approximately 1.63 V(that is, 1.67 V below VDD).

Reference stage 242 includes first and second PMOS FETs M10 and M11having their source-drain paths connected in series with one another andbetween first power supply rail 204 and terminal 234. The gate of FETM10 is connected to terminal 234 (the drain of FET 11) to minimize thevoltage drop across the series-connected source-drain paths of FETs M10and M11. The gate of FET M11 is connected to terminal 236 (the drain ofFET M9), whereby the drain of FET M9 applies voltage VBP2 to the gate ofM11. Main mirror current 224 flows through FET M10 and establishes thegate-source voltage of FET M10, and thus, voltage VBP1 on the gate ofFET M10. The arrangement minimizes the overall voltage drop across thecombined source-drain paths of FETs M10 and M11 while keeping FETs M10and M11 in saturation (similar to the arrangement of FETs M8 and M9).Bias voltage VBP1 has an exemplary value of approximately 2.2 V (thatis, 1.1 V below VDD).

Thus, reference voltage stage 238 can be considered a low-voltagereference stage for establishing bias voltages VBP1/VBP2 in response tocurrents 222/224. Further, since low-voltage reference stage 238establishes bias voltages VBP1/VBP2 in response to bias and main mirrorcurrents 222/224, bias voltages VBP1/VBP2 precisely track input currentI_(IN) over at least process, temperature, and power supply voltagevariations.

PMOS Current Source (244)

Cascode current source 244 includes first and second series-connectedPMOS FETs M12 and M13, connected between power supply rail 204 andterminal 220. Reference voltage stage 238 applies bias voltages VBP1 andVBP2 to the respective gates of FETs M12 and M13, whereby current source244 produces feedback current 226 in response to the bias voltagesVBP1/VBP2. Since bias voltages VBP1/VBP2 precisely track input currentI_(IN), and since current source 244 produces feedback current 226 inresponse to the bias voltages, feedback current 226 also preciselytracks current I_(IN).

Reference Bias Stage (216)

Reference bias stage 216 includes an NMOS FET M6 configured as a diodeand connected in series with an NMOS FET M7, also configured as a diode.Diodes M6 and M7 are connected in series with one another and aretogether connected between second power supply rail 206 and terminal220, so as to produce a voltage drop between the terminal 220 and powersupply rail 206 equal to approximately two diode voltage potentialdrops. Feedback current 226, supplied by current source 244, flowsthrough diodes M6 and M7. In response to feedback current 226, diodes M6and M7 establish voltage VBN2 at the output of the bias stage 216(terminal 220). Therefore, voltage VBN2 can be considered a feedbackvoltage in circuit 106. Since feedback current 226 replicates inputcurrent I_(IN) for all of the reasons described above, and since diodesM6 and M7 establish/derive voltage VBN2 in response to feedback currentI_(IN), voltage VBN2 also tracks current I_(IN). Bias voltage VBN2 hasan exemplary value of approximately 1.33 V.

Reference bias stage 216 applies voltage VBN2 to the respective gates ofcascode FETs M3 and M5. Also, bias and mirror currents 222 and 224flowing through respective FETs M3 and M5 cause respective,corresponding source-gate voltage drops VGS3 and VGS5 in FETs M3 and M5.Since FETs M3 and M5 each have a gate voltage equal to VBN2, FETs M3 andM5 have respective drain voltages VBN2-VGS3 and VBN2-VGS5. VoltagesVBN2-VGS3 and VBN2-VGS5 are applied to the respective drains of FETs M2and M4 by direct connection. Therefore, cascode FETs M3 and M5respectively establish the source-drain voltages of FETs M2 and M4.

Since voltage VBN2 tracks input current I_(IN) via the feedbackmechanism described above, and since voltages VGS3 and VGS5 correspondto respective currents 222 and 224, the present invention controls thesource-drain voltages of FETs M2 and M4 in a dynamic, adaptive manner,such that the drain-source voltages of FETs M2 and M4 are maintainedequal to the source-drain voltage of FET M1 over process, temperature,and power supply voltage variations.

A summarizing example feedback scenario is now provide. Assume inputcurrent I_(IN) is reduced from an initial current value to a reducedcurrent value. In response, the voltage at input terminal 208 (biasvoltage VBN1) is correspondingly reduced, and thus, the gate-sourcevoltages of FETs M2 and M4 are correspondingly reduced. In response,currents 222 and 224 are reduced, and the gate voltages of M8 and M10are directed toward VDD. In response, feedback current 226 is reduced.In response, the voltage drop developed across FETs M6 and M7 isreduced, and thus, the gate voltages of FETs M3 and M5 are reduced. Inresponse, the drain voltages of FETs M2 and M4 are reduced, so theymatch the reduced drain-source voltage of FET M1. Therefore, all of thevoltages and currents track in bias circuit 100.

NMOS and PMOS Current Sources

As discussed in connection with FIG. 1, bias voltages VBN1/VBN2 can beused to control one or more current sources of a first type, such asNMOS current source 110. Cascode current source 110 includes first andsecond series-connected NMOS FETs M16 and M17 having respective gatesdriven by bias voltages VBN2 and VBN1. Current source 110 producescurrent 112 (I_(OUT) _(—) _(N)) in response to bias voltages VBN1/VBN2.Since bias voltages VBN1/VBN2 track input current I_(IN), current 112(I_(OUT) _(—) _(N)) replicates input current I_(IN) over process,temperature, and power supply voltage variations.

Similarly, bias voltages VBP1/VBP2 can be used to control one or morecurrent sources of a second type complementary to the first type, suchas PMOS current sources 244 and/or 120. The operation of PMOS cascodecurrent source 244 was described above, and need not be describedfurther.

Example Implementation

Table 1 below lists the sizes of FETs M1-M17 according to an exampleimplementation of the present invention.

TABLE 1 Device Size FET No. of Devices W/L (μm) M1 192 10/0.35 M2 3210/0.35 M3 32 10/0.5 M4 192 10/0.35 M5 192 10/0.5 M6 32 10/0.35 M7 3210/0.5 M8 2  5/1 M9 4  5/0.5 M10 48  5/1 M11 48  5/0.5 M12 8  5/1 M13 8 5/0.5 M14 8  5/1 M15 8  5/0.5 M16 32 10/0.5 M17 32 10/0.35

Table 2 below lists various current values flowing in circuit 106 in theexample implementation of the present invention.

TABLE 2 Current Label Current Value (μA) Input current I_(IN) 937.5 Biascurrent 222 156.3 Main mirror current 224 937.5 Feedback current 226156.3 PMOS output current 122 156.3 NMOS output current 112 156.3

The FETs depicted in FIG. 2 are connected in a tiered or levelingarrangement, namely:

a first tier includes FETs M1, M2, M4, M6, and M17;

a second tier includes FETs M3, M5, M7, and M16;

a third tier includes FETs M9, M11, M13, and M15; and

a fourth tier includes FETs M8, M10, M12, and 14.

With reference to FIG. 2 and table 1 above, it can be seen that in eachtier (for example, the first tier), the small FETs used to construct allof the aggregate FETs for the tier (for example, M1, M2, M4, and M6 inthe first tier) have the same channel size (for example, W/L=10/0.35microns). On the other hand, the small FETs used to construct aggregateFETs on different tiers do not necessarily have sizes equal to the smallFETs used in the first tier.

With reference to FIG. 2, and Tables 1 and 2 above, it can be seen thatthe aggregate FETs are of such physical transistor dimensions (such asgate length, width and total number of gates) that the current densitiesin the cascode FETs at the second and third tiers (for example, FETs M3and M5, and M9 and M11) are the same as the current densities in thecorresponding current source FETs at the first and fourth tiers (forexample, FETs M2 and M4, and M8 and M10). This further helps thecurrents and voltages within circuit 106 track one another overtemperature and process.

Current Source (102)

FIG. 3 is a circuit diagram of an example input circuit portion 302connected to main circuit portion 106. Input circuit portion 302includes an input resistor R1 connected between first power supply rail204 and input terminal 208, to set the value of input current I_(IN).Input resistor R1 is used instead of input current source 102, discussedabove in connection with FIGS. 1 and 2. Input circuit portion 302 alsoincludes a bypass capacitor C1 connected between input terminal 208 andsecond power supply rail 206. Capacitor C1 reduces noise pick-up andalso the thermal noise generated by the NMOS FETs of circuit 106 (seeFIG. 2). In the integrated circuit embodiment of the present inventionmentioned above in connection with FIG. 2, circuit 106 in constructed onan IC chip. In an arrangement of the integrated circuit embodiment,input resistor R1 and bypass capacitor C1 are external to the IC chip.

Circuit Start-up Feature

FIGS. 4A, 4B and 4C are circuit diagrams of start-up stage or circuit218 according to three different embodiments of the present invention.

With reference to FIG. 4A, a start-up current source 218 a, connectedbetween first power supply rail 204 and input terminal 220, supplies aninitial trickle or leakage current I_(START) to terminal 220, and thusto diodes M6 and M7 so as to bias the diodes on. In doing so, currentsource 218 forces circuit 106 into a proper and stable operatingcondition, that is, to operate as described above. Current source 218 asupplies the initial trickle current (I_(START)) to diodes M6 and M7when bias circuit 100 is initially turned-on. As bias circuit 100 beginsto operate as described above, bias voltage VBN2 at terminal 220 beginsto rise. In response to the rise in voltage VBN2, start-up currentsource 218 a supplies progressively less current (I_(START)) to terminal220. Eventually, start-up current source 218 a supplies no current toterminal 220 (and diodes M6 and M7) when bias circuit 100 attains asteady-state, normal operating condition and when the voltage atterminal 220 rises above ground (VSS).

FIG. 4B is a circuit diagram of another example start-up stage 218 b.Start-up stage 218 b includes a start-up resistor R2 connected betweenpower supply rail 204 and terminal 220. Resistor R2 provides tricklecurrent I_(START) to diodes M6 and M7 so as to bias the diodes on.Resistor R2 supplies current (I_(START)) to diodes M6 and M7 insubstantially the same manner as does start-up current source 218 a,discussed above in connection with FIG. 4A. However, resistor R2continues to supply a tiny trickle current to terminal 220, even afterbias circuit 100 attains the steady-state operating condition mentionedabove. However, the tiny trickle current is sufficiently small as to notdegrade the proper operation of bias circuit 100. Resistor R2 is largeenough that the current I_(START) flowing through it is small comparedto the current 226 from the PMOS current mirror 244. This ensures goodaccuracy in the bias circuit 100.

FIG. 4C is a circuit diagram of yet another example start-up stage 218c. Start-up stage 218 c includes a plurality of, in this case three,series-connected PMOS FETs M18, M19, and M20, having their respectivesource-drain current paths connected in series with each other, andbetween first power supply rail 204 and input terminal 220. All of thegates of FETs M18-M20 are connected to second power supply rail 206(GND). In the depicted configuration, each of FETs M18-M20 operates inits triode region, that is, as a resistor. FETs M18-M20 have relativelylong channels (for example, L/W=0.4 um/10 um), that is, the FETs arerelatively long-channel devices, which are more space-efficient thanresistors, in many cases. Start-up stage 218 c supplies start-up currentI_(START) to terminal 220 in much the same manner as does start-upresistor R2, as described above in connection with FIG. 4B. An addedbenefit is that PMOS FETs M18-M20 tend to turn-off as bias voltage VBN2rises at terminal 220, which as described above, is a desired effect.Turning-off the start-up current I_(START) helps maintain the accuracyof currents and voltages in circuit 106.

Circuit Power-Down Feature

FIGS. 5A-5C are circuit diagrams of three different power-down stagesfor bias circuit 100. Each power-down stage interrupts the flow ofcurrent I_(IN) into circuit 106 to turn-off (that is, “power-down”)circuit 106. With reference to FIG. 5A, a shut-down stage 502 includes aswitch connected to input resistor R1, first power supply rail 204, andsecond power supply rail 206. Switch 502 receives a chip enable/disablecontrol signal 504 from an external control source, not shown. Inresponse to enable/disable states of control signal 504, switch 502selectively connects input resistor R1 to first power supply rail 204 toenable input current I_(IN), and to second power supply rail 206 todisable input current I_(IN). In an alternative arrangement of switch502, the switch is disconnected from first power supply rail 204 andmaintained in an “open” position in response to the disable state ofcontrol signal 504, whereby no current can flow through resistor R1.

With reference to FIG. 5B, a shut-down stage 506 includes an inputcurrent source (corresponding to input current source 102) which can beturned on and off using enable/disable control signal 504.

With reference to FIG. 5C, a shut-down stage 508 includes a switchingFET M20 having a source-drain current path connected between inputterminal 208 and second power supply rail 206, and a gate for receivingenable/disable control signal 504. When control signal 504 correspondsto a logic “1,” FET M20 is turned-on, and thus shunts input currentI_(IN) away from input terminal 208 and toward second power supply rail206. This turns off circuit 106. On the other hand, when control signal504 corresponds to a logic “0,” FET M20 is turned-off, that isnon-conducting, and input current I_(IN) flows into circuit 106. Thisturns on circuit 106.

Another turn-off stage can include a non-inverting buffer, oralternatively an inverting buffer, having an input driven by a controlsignal having an appropriate polarity and an output connected to the endof resistor R1 connected to first power supply rail 204.

Methods

FIG. 6A is a flow chart of an example method 600 of establishing firstand second bias voltages (and corresponding mirrored currents) from aninput current implemented using bias circuit 100. Method 600 includes aninitial step 605 of supplying an input current (for example, currentI_(IN)) to circuit 106.

Method 600 includes a next step 610 of establishing a first bias voltage(for example, bias voltage VBN1) in response to the input current.

Method 600 includes a next step 615 of producing a bias current (forexample, current 222) proportional to the input current in response tothe first bias voltage (for example, bias voltage VBN1) and a secondbias voltage (for example, bias voltage VBN2).

Method 600 includes a next step 620 of producing a main mirror current(for example, current 224) proportional to the input current in responseto the first bias voltage and the second bias voltage.

Method 600 includes a next step 625 of producing a feedback current (forexample, current 226) proportional to the input current in response tothe bias current and the main mirror current.

Method 600 includes a next step 630 of establishing the second biasvoltage in response to the feedback current, whereby the first andsecond bias voltages track the input current over variations in at leastone of process, temperature and power supply voltage.

FIG. 6B is a flow chart expanding on method step 625 mentioned above inconnection with FIG. 6A. Step 625 includes a first step 640 ofestablishing third and fourth bias voltages (for example, bias voltagesVBP1, VBP2) in response to the bias current and the main mirror currentproduced in previous steps 615 and 620.

Step 625 includes a next step 645 of producing the feedback current inresponse to the third and fourth bias voltages.

FIG. 6C is a flow chart of an example method 650 further expanding onmethod 600. Method 650 includes a first method step 655 (correspondingto steps 610 and 630 of method 600) of establishing the respective firstand second bias voltages (for example, VBN1/VBN2) such that the firstand second bias voltages are suitable for biasing one or more currentsources of a first type (for example, NMOS current sources).

Method 650 includes a second method step 660 (corresponding to steps 640mentioned above) of establishing the third and fourth bias voltages (forexample, bias voltages VBP1/VBP2) such that the third and fourth biasvoltages are suitable for biasing current sources of a second typecomplementary to the first type (for example, PMOS current sources).

FIG. 6D is a flow chart of an example method 670 of initiallyestablishing or starting-up the proper operation of bias circuit 100.Start-up method 670 includes a first method step 675 of supplying atrickle/leakage current (for example, I_(START)) to establish a stableoperating condition of the bias circuit 100. Method 670 includes anoptional next step 680 of reducing the trickle/leakage current from aninitial current value to a final current value in response to a rise inthe second bias voltage (for example, VBN2) indicative of a stable,proper operating condition of bias circuit 100.

FIG. 6E is a flow chart of an example method 685 of selectively enablingand disabling bias circuit 100. Method 685 includes the step ofselectively enabling and disabling the operation of bias circuit 100 byselectively enabling and disabling the input current (for example,I_(IN)) in response to an enable/disable signal

Conclusion

While various embodiment of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. Thus, the breadth and scope of thepresent invention should not be limited by any of the above-describedexemplary embodiments and arrangements, but should be defined only inaccordance with the following claims and their equivalents.

The present invention has been described above with the aid offunctional building blocks and circuit diagrams illustrating theperformance of specified functions and relationships thereof. Theboundaries of the functional building blocks have been arbitrarilydefined herein for the convenience of the description. Alternateboundaries can be defined so long as the specified functions andrelationships thereof are appropriately performed. Any such alternateboundaries are thus within the scope and spirit of the claimedinvention. One skilled in the art will recognize that these functionalbuilding blocks can be implemented using discrete circuit components,circuit components constructed on an IC chip, or any combinationthereof. Thus, the breadth and scope of the present invention should notbe limited by any of the above-described exemplary embodiments, butshould be defined only in accordance with the following claims and theirequivalents.

What is claimed is:
 1. A circuit adapted to be responsive to an inputcurrent supplied to an input terminal of the circuit, comprising: aninput stage adapted to establish a first bias voltage at the inputterminal in response to the input current; a current stage adapted toproduce at least one mirror current proportional to the input current inresponse to the first bias voltage and a second bias voltage; a feedbackstage adapted to produce a feedback current proportional to the inputcurrent in response to the at least one mirror current; and a referencebias stage adapted to establish the second bias voltage in response tothe feedback current from the feedback stage, whereby the first andsecond bias voltages track the input current.
 2. The circuit of claim 1,wherein the at least one mirror current includes a bias current and amain mirror current.
 3. The circuit of claim 2, wherein the feedbackstage includes: a reference voltage stage adapted to establish third andfourth bias voltages in response to the bias current and the main mirrorcurrent; and a current source adapted to produce the feedback current inresponse to the third and fourth bias voltages.
 4. A bias circuit forestablishing bias voltages from an input current supplied to an inputterminal of the bias circuit, comprising: input stage means forestablishing a first bias voltage at the input terminal in response tothe input current; current stage means for producing a bias current anda main mirror current each proportional to the input current in responseto the first bias voltage and a second bias voltage; feedback stagemeans for producing a feedback current proportional to the input currentin response to the bias current and the main mirror current; andreference bias stage means for establishing the second bias voltage inresponse to the feedback current, whereby the first and second biasvoltages track the input current.
 5. The circuit of claim 4, furthercomprising means for setting a value of the input current supplied tothe input terminal.
 6. The circuit of claim 4, further comprisingstart-up stage means for providing a trickle-current to the referencebias stage to force the bias circuit into a stable operating condition.7. The circuit of claim 4, further comprising shut-down stage means forselectively enabling and disabling the supply of the input current tothe input terminal so as to selectively enable and disable an operationof the bias circuit, respectively.
 8. A method of establishing biasvoltages suitable for biasing current sources from an input currentsupplied to a bias circuit, comprising: (a) receiving the input current;(b) establishing a first bias voltage in response to the input current;(c) producing at least one mirror current proportional to the inputcurrent in response to the first bias voltage and a second bias voltage;(d) producing a feedback current proportional to the input current inresponse to the at least one mirror current; and (e) establishing thesecond bias voltage in response to the feedback current of step (d),whereby the first and second bias voltages track the input current. 9.The method of claim 8, wherein step (c) comprises: (c)(i) producing amain mirror current proportional to the input current in response to thefirst bias voltage and the second bias voltage; and (c)(ii) producing abias current proportional to the input current in response to the firstbias voltage and the second bias voltage.
 10. The method of claim 9,wherein step (d) comprises: (d)(i) establishing third and fourth biasvoltages in response to the bias current and the main mirror current;and (d)(ii) producing the feedback current in response to the third andfourth bias voltages.
 11. The method of claim 10, wherein: step (b) andstep (e) together comprise establishing the first and second biasvoltages such that the first and second bias voltages are suitable forbiasing one or more current sources of a first type; and step (d)(i)comprises establishing the third and fourth bias voltages such that thethird and fourth bias voltages are suitable for biasing current sourcesof a second type complementary to the first type.
 12. The method ofclaim 8, further comprising: supplying a trickle current to establish astable operating condition of the bias circuit.
 13. The method of claim12, further comprising: reducing the trickle current from an initialcurrent value to a final current value in response to a rise in thesecond bias voltage indicative of the stable operating condition. 14.The method of claim 8, further comprising: selectively enabling anddisabling the circuit by selectively enabling and disabling the inputcurrent in step (a) in response to an enable/disable signal.